1. Field of the Invention
This invention relates to a device for measuring currents in a converter fitted with switches controlled by a control circuit and powered by a DC power source.
2. Discussion of Background
A frequency converter is used to control a motor at a variable frequency and voltage, from the AC network. This type of frequency converter consists of a DC power source (rectifier stage) that, after filtering, powers a voltage converter. The converter includes "switches" (for example IGBT transistors) that are controlled by a control circuit with a microcontroller using a technique called pulse width modulation (or PWM) outputting a sequence of fixed amplitude width modulated pulses to the motor. Transistor closing instants are fixed by the intersections of a sinusoidal reference wave representing the output voltage from a phase and a triangular modulation wave.
In a frequency converter, it is necessary to measure the current in the phases in order to regulate the speed and to provide protection against overcurrents and short circuits. There are several methods of measuring the current. A first method is to place Hall effect sensors or shunts with insulating amplifiers directly on the phases. A second method consists of placing a single shunt on the return conductor to the DC power source and "reconstructing" output currents in the phases. A third method consists of placing three shunts on the lower arms of the converter and "reconstructing" the output currents in the phases, making use of the voltage measurements at the terminals of the three shunts.
Known measurement methods using three shunts in the lower branches are not fully satisfactory, particularly at high speed and in defluxed mode.